Cross-linked glucose isomerase

ABSTRACT

The invention relates to a novel water-insoluble glucose isomerase which is formed by a crystalline enzyme converted to solid form by cross-linking. The invention also concerns a process for the preparation of the novel crystalline glucose isomerase by cross-linking with dialdehyde in the presence of a compound containing at least one amino group, and the use of this novel enzyme preparation as an isomerization catalyst.

This is a Division of application Ser. No. 08/149,158 filed Nov. 8,1993, now U.S. Pat. No. 5,437,993, which is a continuation ofapplication Ser. No. 07/974,371, filed Nov. 10, 1992, now abandoned,which is a division of application Ser. No. 07/350,720, filed May 11,1989, now abandoned.

FIELD OF THE INVENTION

This invention relates to a novel water-insoluble glucose isomeraseformed by cross-linking of the crystalline enzyme. The invention is alsoconcerned with a process for the preparation of the insoluble,cross-linked glucose isomerase.

BACKGROUND OF THE INVENTION

The use of immobilized enzymes, i.e., enzymes bound to a solid carrier,in continuously operated reactors is an increasingly preferredtechnique, since it enables savings in enzyme costs as well as in thepurification of the final product. The conversion of glucose to fructosewith an immobilized glucose isomerase is a process that commonlyutilizes such a procedure.

In general, enzymes are water-soluble, thus making it necessary to usean immobilization technique in a continuous process. The enzyme has tobe bound to a solid phase by a method that prevents it from dissolvingin the aqueous phase while allowing it to maintain its activity. Varioustechniques have been suggested to associate enzymes with solid carrierswherein the enzyme is absorbed, covalently linked, cross-linked ormicroencapsulated. Alternatively, the entire microorganism producing theenzyme can be bound to a solid phase. A good summary of such techniquesis presented in e.g. Moo-Young, M. (ed), Comprehensive Biotechnology, 2,Pergamon Press, London 1985, p. 191-211.

In prior processes, the enzyme is bound to a carrier material preparedseparately, which as such may be advantageous to the chemical kineticsor the flow technique of the substrate. The carrier material is,however, in most cases more costly than the enzyme acting as a catalyst,especially in large-scale mass production processes, such as those usedin the sugar industries. Alternatively, the enzyme can be immobilized bycross-linking it with an inert component, such as gelatin. In any case,the enzyme acting as a catalyst in the prior art forms only a fraction,generally less than 5%, usually 1 to 2%, of the weight and volume of thematerial used in each particular process.

Other techniques that have been applied include linkage to ionexchangers and absorption to a solid carrier. An example of such anapplication can be found in U.S. Pat. No. 4,699,882. However, thecarrier used in this prior art technique is relatively expensive and thetechnique requires a large reactor.

The immobilization of enzymes for industrial use entails costs which arenot necessarily associated with the used enzyme; they include costscaused by the construction of the process apparatus and the factorypremises, carrier material acquisition (reacquisition) costs, cost ofdisposing of inactivated enzyme material, labor costs caused by emptyingand filling reactors (or by the regeneration of the carrier) andsecondary costs caused by the slowness of the reactors. As a consequenceof long retention times, non-enzymatic, disadvantageous side reactionscan often occur, particularly in the production of fructose.

Technically, cross-linking with glutaraldehyde has been of greatimportance in the immobilization of glucose isomerase. As is known,glutaraldehyde is approved by the FDA for the immobilization of enzymesto be used in food processing.

In addition, the scientific literature includes several examples of thecross-linking of crystalline enzymes by means of glutaraldehyde forbasic research purposes. The structure of enzyme crystals is often soweak that the crystals do not withstand the ray beam used in X-raydiffraction studies; however, with glutaraldehyde they can often bestabilized for such purposes. Furthermore, crystals have beencross-linked with the purpose of studying stability and catalysiskinetics. In cases where the cross-linking of crystals has beensuccessful, only glutaraldehyde has been used and the medium hasconsisted of a solution in which each particular enzyme is maintained incrystalline form. It appears that an insoluble crystal has been formeddirectly by a reaction between the glutaraldehyde and the enzymeprotein. Quiocho and Richards (Proc. Natl. Acad. Sci. (USA) 52 (1964) p.833 and Biochemistry 5 (1966) p. 4062) were the first to useglutaraldehyde in the cross-linking of carboxypeptidases. Bishop andRichards (J. Mol. Biol. 33 (1968) p. 415-421) have cross-linkedcrystalline beta lactoglobulin with a 1% aqueous solution ofglutaraldehyde at room temperature. The crystals were used for studyingthe electrical properties of the enzyme. Haas (Biophysic. Journ. 8(1968) p. 549-555) has cross-linked lysozyme crystals in the presence ofa 4% sodium nitrate solution (pH 8), using a glutaraldehydeconcentration of 12%.

Dyer, Phillips and Townsend (Thermochimica Acta 8 (1974) p. 456-464)have studied the thermostability of a crystalline carboxypeptidasecross-linked by glutaraldehyde. They have found that cross-linking leadsto increased stability. Tuechsen and Ottesen (Carlsberg Res. commun. 42(1977) p. 407-420) have studied the kinetic properties of a crystallinesubtilisin cross-linked by glutaraldehyde in a sodium sulfate solution.With low-molecular substrates, the activity of the crystals was highwhereas with high-molecular substrates (that could not diffuse into thecrystals) the activity was low.

Wong et al. (Biochem. and Biophysic. Research Communications 80 (1978)p. 886-890) have cross-linked an acidic protease of microbial originwith glutaraldehyde in an ammonium sulfate solution. In thecross-linking, the presence of ammonium sulfate was regarded as atechnical disadvantage.

Morozov and Morozova (Biopolymers 20 (1981) p. 451-467) havecross-linked crystalline lysozyme, hemoglobin and myoglobin using 2 to6% glutaraldehyde solutions and a reaction time of 2 to 10 days at roomtemperature. Lee et al. (Bioorganic Chemistry 14 (1986) p. 202-210) havecross-linked crystals of alcohol dehydrogenase with glutaraldehyde inthe presence of 2-methyl-2,4-pentanediol (25%).

It is often difficult to produce insoluble enzymes by means ofglutaraldehyde, especially if the protein contains relatively littlelysine. This problem is often circumvented by mixing into the enzyme aprotein such as albumen which can be cross-linked to produce aninsoluble form (G. B. Broun, Methods in Enzymology, 44 (1976) p. 263).The addition of such an inert foreign protein is not, however, possiblewhen the enzyme to be cross-linked is crystalline. To date, there havebeen no means of cross-linking in cases where it is not possible tocross-link a crystal to insoluble form by means of glutaraldehyde only.

SUMMARY OF THE INVENTION

As mentioned above, it is known to immobilize glucose isomerase withglutaraldehyde. The isomerase is thereby cross-linked to a supportmaterial. Such processes are fully utilized industrially. Attempts tocross-link glucose isomerase crystals to insoluble form, however, arenot described in the literature.

It has now been found that it is possible to cross-link glucoseisomerase crystals in such a way that the original crystalline state ismaintained while the enzymatic activity of the enzyme remains very high.Optimally, the crystalline enzyme has the same activity as that of theoriginal enzyme. The product according to the invention is not solublein any solvents that might be present when using the enzyme technically.Cross-linked crystalline enzyme can be used as such to fill anisomerization column in a technical isomerization process. By means ofthe novel cross-linked crystalline enzyme it is possible to carry out amore efficient, continuous isomerization process in columns much smallerthan used previously in industrial processes. This is because thepresent invention enables use of a column filled with pure enzyme,rather than enzyme bound to an inert material which often occupies amajority of the space of the column and accounts for a majority of itscost.

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention, glucose isomerase crystalsare cross-linked by means of a dialdehyde, such as glutaraldehyde, and acompound containing at least one amino group, such as an ammoniumcompound, amine, or amino acid, preferably an ammonium salt or lysine.Several amines and amino acids are suitable for use. It is likewiseevident that in addition to glutaraldehyde, many other dialdehydes andsubstances reacting with amino groups, known as cross-linking reagents,can be used in the process.

The activity of a solid crystalline enzyme prepared by means of theprocess according to the invention is very high. Depending on theconditions used for its production, its activity may be almost the sameas that of a free enzyme. Solid crystalline enzyme can be used as suchas a column filler in a continuous process. It is extremely stable andwithstands mechanical stress well.

If desired, cross-linked crystals can be further bound to form largerbodies, e.g. in spherical, sheetlike, bandlike or the like in shape, inknown chemicophysical ways. Enzyme preparations so obtained canwithstand various mechanical treatments.

Those skilled in the art will recognize that the cross-linking process,as disclosed herein, can be used to prepare insoluble preparations ofvirtually any crystalline enzyme which has not, generally because of itsamino acid composition, heretofore been found to be conducive tocross-linking.

In the following, the process according to the invention will bedescribed in more detail.

Preparation of crystalline glucose isomerase used as raw material.

Ammonium sulfate (about 10% by weight) is dissolved in a glucoseisomerase solution (1 to 10% by weight of isomerase determined as dryprotein). The solution is cooled slowly to about 1 to 2° C. underconstant stirring. An essentially complete crystallization of theisomerase thereby occurs (more than 95%). In place of ammonium sulfate,magnesium sulfate or sodium sulfate, for instance, can be used as acrystallization agent. The concentration of the salts used may varywithin wide limits, e.g., from 5 to 25% by weight. The time required forthe crystallization process also varies within wide limits, e.g., fromone hour to several days. In the preparation of large crystals, it ispreferable to apply gradual cooling, and the isomerase should be as pureas possible. The crystallization is described in U.S. Pat. No. 4,699,882(Visuri) which is incorporated herein by reference.

After crystallization the crystal mass is separated by sedimentation orby centrifuging from the mother liquor thereof. If required, the crystalmass is washed with pure solutions of ammonium sulfate, magnesiumsulfate or some other substance suitable for crystallization.Cross-linking is carried out on a crystal mass which does not containany free excess mother liquor and which is sedimented or centrifuged tocompact form. A typical activity of such crystal mass is 10,000 GIU/g.It contains 20 to 30% by weight of pure enzyme protein determined as drysubstance. It should be noted that enzyme crystals lose their structureif they are dried.

Cross-linking.

Crystal mass is suspended in a salt solution such that the crystals willnot dissolve therein. The concentration of the enzyme crystals in thesolution may vary widely, e.g., 2 to 17% by weight on dry substance.

Ammonium salt is added to the solution if the salt solution does notinitially contain ammonium, or a suitable amine or amino acid, such aslysine, is added. The pH of the mixture is adjusted to a value from 5 to9, preferably from 6 to 8, by adding, for instance, sodium hydroxidesolution. Acidity is preferably controlled by a phosphate buffer, e.g.,0.05M sodium phosphate or by automatic pH control with an alkalinesolution such as sodium hydroxide. The useful concentration of the addedamine or amino acid varies within wide limits and is dependent on theconcentration of the other components. The product according to theinvention has been prepared with a high yield with amine or amino acidconcentrations of 1 to 15% of the final weight of the mixture.

Thereafter glutaraldehyde is added to the mixture for initiating thecross-linking reaction. The amount of glutaraldehyde can be variedwithin wide limits, from 1 to 45% by weight on wet enzyme crystal mass.The preferred amount a depends e.g. on the concentration of the aminecontained in the mixture. In general, the preferred concentration variesfrom 3 to 4.5 g of glutaraldehyde per 3 g of isomerase calculated as dryenzyme. During the reaction the solution is stirred continuously; thetemperature ranges from 2 to 25° C. A low temperature is of advantagethough the temperature does not seem to be critical. The reaction mayoccur very rapidly, in a few minutes, especially at higher temperatures.At a low temperature the reaction time may be up to 20 hours.

After the reaction, the insoluble crystals are separated from themixture by sedimentation or centrifugation. The crystal mass is washedby suspending in water or a suitable salt solution and byrecentrifugation. The washing is repeated several times until thecrystal mass is sufficiently pure to be used as a catalyst. Inconnection with the washing it is also preferable to flush offfine-grained precipitate.

It is not advisable to dry the obtained crystal mass if it is to be usedas a catalyst in an enzymatic process. Wet crystal mass fully retainsits activity for at least six months without any special measures.

Characterization of the final product.

Cross-linked crystals prepared according to the invention are similar tothe original raw material crystals in appearance and size. The size ofthe crystals is not critical. Crystals having a diameter of 100 to 200micrometres or larger (up to 1 millimeter) are particularly suited fortechnical use.

The most important property of the crystals according to the inventionis that they are insoluble in water, salt solutions and sugar solutions.It is of particular importance that the isomerase crystals do notdissolve in concentrated solutions of glucose and fructose, even at hightemperatures. In industrial processes it is customary to use atemperature of 60° C. and a sugar concentration of 45% by weight.Cross-linked crystals are insoluble under such conditions and at anyother sugar concentration and higher temperatures (up to 100° C.). Theactivity of cross-linked crystals may be within an order of magnitude ofthe activity of the original enzyme. Technically, it is an advantage andof importance that their activity is many times higher than that of anenzyme immobilized on an inert carrier.

On cross-linking an enzyme in crystalline form, a further advantage isobtained in that the enzyme is stabilized to a notable extent by forcesacting in the crystal and naturally keeping together the crystal.

Methods used for the characterization of The starting material and thefinal product.

The activity of the isomerase was determined as international glucoseisomerase units, abbreviated GIU, per 1 g of a dried enzyme preparation.One unit (GIU) represents an enzyme amount able to convert glucose tofructose at a rate of 1 micromole/min under the following conditions:glucose concentration 2.0 mole/liter, pH 7.0, and temperature 60° C.

For the activity determination, 0.1 to 1 g of an enzyme preparation(original crystal mass or thoroughly washed cross-linked crystal mass)was mixed into a substrate solution (100 ml) such as described above.After a suitable period of time, e.g., 10 minutes, the fructose contentof the solution was determined and the activity was calculated andexpressed in the above-mentioned units. The amount of enzyme and thereaction time were chosen so that the fructose formed was less than 5%of the total sugar content in order that the measuring result wouldconcern the initial rate of the reaction. The dry content of thestarting material and the product were determined by a conventionalmethod by drying the sample at 105° C. to constant weight.

Isomerization process.

Crystalline cross-linked enzyme is suitable for use in a conventionalway in a batch isomerization process, whereby the used enzyme isseparated after the reaction, e.g., by filtration, and it can be reused,if desired.

On an industrial scale, however, it is to be preferred to carry out theisomerization as a continuous process, whereby the sugar solution to beisomerized is allowed to flow through an enzyme column. By varying theretention time and/or the temperature, the isomerization process is easyto adjust. The enzyme is active within a wide range of temperatures,from the freezing point up to temperatures exceeding 100° C. At lowtemperatures, a drawback is that the reaction is slow and the sugar(glucose and fructose) hydrates are crystallized, whereas at hightemperatures the destruction of both the enzyme and the fructose takesplace considerably more rapidly.

Continuous isomerization is typically carried out in such a manner thatthe column is filled with cross-linked enzyme crystals of 100 to 300micrometres. The size and height of the column can be varied accordingto the capacity required in each particular case. In a small column asuitable bed height is 5 to 50 cm. The temperature may also be variedwithin wide limits. Room temperature is readily realizable. Ifmicrobiological contaminations present a problem, they can be eliminatedby a rising the temperature to at least about 60° C.

The process is easy to control by varying the linear flow rate. With asmall column the linear flow rate ranges from 2 to 30 cm/min. Aretention time suitable for fresh enzyme is 1 to 2 minutes. The pressureis atmospheric. The pressure loss is insignificant (<0.2 bar per a bedheight of 50 cm). The retention time is adjusted by varying the bedheight of the column and by the flow rate. The isomerization reactioncan be accelerated by raising the temperature.

In a conventional industrial process, the aim is, in most cases, toobtain a sugar solution in which 40 to 45% of the sugar is fructose.When the activity of the enzyme decreases with the ageing of the column,the desired level is maintained by reducing the flow rate.

The following examples illustrate the invention more closely:

EXAMPLE 1

850 g of glucose isomerase crystal mass crystallized in a 10% ammoniumsulfate solution as described in U.S. Pat. No. 4,699,882 was weighed andto said crystal mass, 1000 ml of a sulfate solution pH 7.4 (buffered bya 0.5M sodium phosphate) was added. The mixture was cooled to 10° C. Theresulting mixture was stirred constantly by means of a propeller stirrerusing a low speed (200 to 400 rpm), for decreasing the damaging of thecrystals. A 160 ml aliquot of 25% glutaraldehyde was added to themixture. After one hour the reaction was arrested by adding 20 liters ofpure water into the mixture. The stirring was ended immediately and thecrystal mass was allowed to settle on the bottom of the vessel for twohours. The mother liquor was decanted apart taking care not to flush offthe crystals. 20 liters of pure water were again mixed into the crystalmass and the washing water was removed by decanting. Still anotherwashing with water was carried out. The obtained wet isomerase crystalmass, washed three times with water, was used as such in isomerizationtests, activity determinations and other experiments.

The cross-linked glucose isomerase so prepared was in a crystallinestate (microscopic appreciation). The crystals were insoluble in water,dilute solutions of various salts (within the pH range of 2 to 9),dilute acids (1 mole/liter), hot water and hot salt solutions up to 100°C., and concentrated glucose, fructose and sugar solutions up to 100° C.The appearance of the crystals remained unchanged under all theabove-mentioned conditions, whereas crystalline isomerase which had notbeen cross-linked was dissolved or precipitated as an amorphousprecipitate. The enzymatic activity of the cross-linked isomerase was52% of that of the original isomerase which had not been cross-linked.That is, when the activity of the original isomerase was 40,000 GIU/g,the activity of cross-linked crystals was correspondingly more than20,000 GIU/g, calculated per dried enzyme protein.

EXAMPLE 2

With the arrangement of Example 1, the following reaction mixture wasprepared at 25° C.:

14 g isomerase calculated as pure enzyme protein

10 g ammonium sulfate

1.5 g glutaraldehyde (about 8 ml of a 25% solution)

0.05M sodium phosphate buffer (pH of the solution 7.4)

100 ml water

After a reaction time of one hour, free solution was removed from themixture by means of a laboratory centrifuge by centrifuging at 1000 rpmfor five minutes. The crystal mass obtained was suspended in 200 ml ofpure water and centrifuged again as described above. Washing with waterwas repeated once more as described above. Wet washed crystal mass wasrecovered and used for further research. The activity of the crystalmass so prepared was 49% of that of the original crystal mass.

EXAMPLE 3 TO 8

Reaction mixtures having the same initial composition as in Example 2were prepared with the arrangement of Example 1 at 10° C. After reactiontimes of varying lengths, the reaction was arrested and the crystal masswas washed, whereafter the activity of each crystal mass was determined.The results are shown in the following Table 1.

                  TABLE 1    ______________________________________                       Activity (% of the                       activity of the              Reaction time                       original crystal mass)    ______________________________________    Example 3   10 min     45    Example 4   30 min     46    Example 5   60 min     49    Example 6   90 min     48    Example 7    2 h       40    Example 8    3 h       40    ______________________________________

It can be seen from the results that the reaction is completed veryrapidly and the activity does not change in any greater degree when thereaction time is increased.

EXAMPLES 9 TO 16

With the arrangement of Example 1, reaction mixtures having thefollowing initial composition were prepared at 10° C.:

14 g glucose isomerase protein (in crystalline form)

10 g ammonium sulfate

90 ml 0.5M sodium phosphate solution (pH 6.0,7.0, 8.0 or 8.4, as shownin Table II)

glutaraldehyde 0.12, 0.5, 2.0, 3.5 or 4.12 g (as shown in Table II)

Reaction time was one hour. The activity of each resultant crystal mass(% on the original activity of the crystal mass) is shown in Table II.

                  TABLE II    ______________________________________                      Glutar    Activity             pH       aldehyde (g)                                (%)    ______________________________________    Example 9  6.0        0.5       22    Example 10 6.0        3.5       24    Example 11 7.0        0.12      60    Example 12 7.0        2.0       65    Example 13 7.0        4.12      59    Example 14 8.0        0.5       41    Example 15 8.0        3.5       44    Example 16 8.4        2.0       39    ______________________________________

It appears from the results that the amount of glutaraldehyde can bevaried within fairly wide limits and nevertheless obtain highactivities. The acidity greatly affects the result; the preferred pHvalue is about 7.0, though useful preparation can be obtained within theentire pH range tested, i.e., pH 6.0 to 8.4.

EXAMPLES 17 TO 27

A test series similar to the preceding examples was carried out;however, the temperature was 2° C. and the reaction time 18 hours. Thecomposition of the reaction mixture was the following:

3 g isomerase crystals calculated as dry protein

50 ml water as a medium

7.5 g salt (sodium sulfate, magnesium sulfate and/or ammonium sulfate(see Table III)

sodium hydroxide for adjusting pH to 7.0 (not more than 2 meq, that is,80 mg)

0.125 to 2.5 g glutaraldehyde (see Table III).

                  TABLE III    ______________________________________                                    Glutar-                  Salt (g)          aldehyde                                           Activ-    (NH4)2SO4     MgSO4    Na2SO4   (g)    ity (%)    ______________________________________    Example 17                 7.5    0.5    0    Example 18                 7.5    2.5    0    Example 19        7.5             0.125  0    Example 20        7.5             0.5    0    Example 21        7.5             0.75   0    Example 22        7.5             1.25   0    Example 23            0.45      7.05            0.75   20    Example 24            0.90      6.6             0.75   19    Example 25            1.8       5.7             0.75   28    Example 26            3.75      3.75            0.75   40    Example 27            7.5                       0.75   60    ______________________________________

It appears from the results that when cross-linking is carried out inthe absence of an ammonium salt, insoluble crystals are not obtained,not even with a high glutaraldehyde concentration. Even a small amountof ammonium salt promotes the formation of insoluble crystals.

EXAMPLES 28-38

Insoluble isomerase crystals were prepared using various nitrogencompounds according to the following general method:

3 g crystalline isomerase calculated as dry protein

7.5 g magnesium sulfate

10 mmol nitrogen compound (see Table IV)

2.5 g glutaraldehyde (calculated as 100%)

temperature 2° C. and

reaction time 18 hours

                  TABLE IV    ______________________________________             Nitrogen             compound  Amount (g)                                 Activity (%)    ______________________________________    Example 28 lysine      1.83      80    Example 29 arginine    1.74      17    Example 30 histidine   1.55      26    Example 31 glutamine   1.46      18    Example 32 leucine     1.31      21    Example 33 isoleucine  1.31      18    Example 34 proline     1.15      10    Example 35 methionine  1.49      27    Example 36 phenylalanine                           1.65      17    Example 37 tryptophane 2.04      44    Example 38 betaine     1.17      23    ______________________________________

It appears from the results that a number of different amines have asimilar effect on the cross-linking process.

EXAMPLES 39 TO 51

Insoluble isomerase crystals were prepared with different dosages ofglutaraldehyde and lysine according to the following general precept:

3 g crystalline isomerase calculated as dry protein

7.5 g magnesium sulfate

0.47 to 2.34 glycine (see Table V)

pH adjusted to 8.0 with a sodium hydroxide solution

0.5 to 1.25 g glutaraldehyde (100%; see Table V)

Solution was allowed to react 18 hours at 2° C.

                  TABLE V    ______________________________________             Glutar      Lysine  Activity             aldehyde (g)                         (g)     (%)    ______________________________________    Example 39 0.5           0.47    67    Example 40 0.5           0.94    77    Example 41 0.5           1.40    60    Example 42 0.75          0.47    72    Example 43 0.75          0.94    83    Example 44 0.75          1.40    92    Example 45 0.75          1.87    81    Example 46 0.75          2.34    75    Example 47 1.25          0.47    68    Example 48 1.25          0.94    78    Example 49 1.25          1.40    90    Example 50 1.25          1.87    102    Example 51 1.25          2.34    100    ______________________________________

It appears from the results that the ratio between the dosages of lysineand glutaraldehyde affects the formation of insoluble crystals, that is,an optimum lysine dosing level is to be seen at each glutaraldehydedosing level.

EXAMPLE 52 to 56

Isomerase crystals were cross-linked in the solutions of ammoniumsulfate and lysine according to the following general precept:

10 g isomerase crystals in 10% ammonium sulfate (i.e. 3.72 g pureisomerase protein calculated as dry substance, 0.63 g ammonium sulfateand 5.65 g water)

5 g ammonium sulfate

45 g water

1.25 g glutaraldehyde calculated as 100%

0 to 3 g lysine (see Table VI) The pH of all the reaction components wasadjusted to 8.0 by means of sodium hydroxide before stirring.

The mixtures were stirred at 3° C. for 18 hours.

                  TABLE VI    ______________________________________                 Lysine (g)                        Activity (%)    ______________________________________    Example 51     0        40    Example 52     0.3      62    Example 53     0.6      66    Example 54     1.2      72    Example 55     1.8      92    Example 56     3.0      96    ______________________________________

It appears from the results that lysine affects very favorably the yieldof the cross-linking. Ammonium sulfate has no greater effect on theresult when lysine is available, even though ammonium sulfate alonegives a satisfactory result.

EXAMPLES 57 to 68

The amount of isomerase and lysine was kept constant and the othercomponents were varied as shown in Table VII:

3 g glucose isomerase calculated in dry form

1 g lysine

0.01 g sodium hydroxide (solution pH 8).

The reaction time was 18 and temperature 2° C.

                  TABLE VIII    ______________________________________    Glutar                         React. Activ-    aldehyde     MgSO4    Water    solution                                          ity    (g)          (g)      (g)      total (g)                                          (5)    ______________________________________    Example 57            0.5      2.1      11.9   18.5   39    Example 58            0.5      2.7      15.3   22.5   70    Example 59            0.5      4.2      23.8   32.5   70    Example 60            0.5      5.3      32.7   42.5   73    Example 61            0.5      10.2     57.8   72.5   75    Example 62            0.5      16.2     91.8   112.5  77    Example 63            1.25     2.1      11.9   19.25  86    Example 64            1.25     2.7      15.3   23.25  99    Example 65            1.25     4.2      23.8   33.25  89    Example 66            1.25     10.2     57.8   43.25  83    Example 68            1.25     16.2     91.8   113.25 89    ______________________________________

It appears from the results that the concentration of the reactionmixture has little effect on the final result. The weight ratios betweenthe reaction components (enzyme, lysine (or amine) and glutaraldehyde)have a much greater effect.

EXAMPLE 69

1 g of cross-linked glucose isomerase mass (from Example 55; 0.4 g drysubstance) washed with water was mixed into 100 g of a 40% glucosesolution the pH of which had been adjusted to 7.0. The mixture wasstirred constantly at 60° C. Samples were taken from the mixtureintermittently, and the fructose content of the samples was determinedby means of a polarimeter and the glucose content measured enzymaticallyby means of a hexokinase. The fructose content of the solution rose to42% on the total sugar content of the solution (glucose+fructose=100%)during 3 hours. After the test the cross-linked isomerase was separatedfrom the mixture by filtering and was washed with water. The recoveredcrystal mass was tested for its activity and dry content and it wasfound that no active enzyme had been dissolved or disappeared in thetest. The test could be repeated several times with the same enzymebatch.

EXAMPLE 70

Crystal mass prepared as described in Example 1 was washed for removingfine-grained precipitate and crystal material crushed fine during theprocess by suspending in water and decanting (3 times). The largecrystal fraction so obtained, mean size 100 micrometres, was packed intoa cylindrical reactor having a diameter of 2.6 cm and a height of 5 cm.Glucose solution having the following composition was pumped through thecolumn at 60° C.:

582 g glucose monohydrate

590 g water

0.37 g M_(g) SO₄.7H₂ O

0.19 g NaHS03

pH 6.9 (1M NaOH, consumption below 1 ml) At the beginning of the testthe flow rate was 11 ml/min, whereby the fructose content of thesolution discharged from the column had risen to 42% on the total sugarcontent. The test was continued for 200 hours, whereafter the flow ratehad to be reduced to 9 ml per minute for maintaining the originalconversion (fructose content 42%). Accordingly, the activity of theenzyme had dropped during this period of time to 81% from the initialvalue. No reduction or dissolving of the crystal mass could be observedduring the test.

EXAMPLES 70 to 74

43.42 g of wet active isomerase mass washed with water (prepared by themethod of Example 66; 10.0 g enzyme on dry substance) was weighed.

200 ml of a glucose solution prepared as described in Example 70 waspoured on the crystal mass. The mixture was shaken at 60° C. fordifferent periods of time and the mixture was then filtered through afilter paper disc. The crystal mass remaining on the paper was washedcarefully with water for removing all soluble material. The crystal masswas dried in an incubator at 105° C. and weighed. The observations arepresented in the following Table VIII:

                  TABLE VIII    ______________________________________                       Dry weight of crystal              Stirring Time                       mass after test (g)    ______________________________________    Example 70  10 min     9.9    Example 71   2 h       11.0    Example 72   4 h       10.9    Example 73   6 h       10.7    Example 74  21 h       10.4    ______________________________________

It appears from the results that the cross-linked crystalline isomeraseprepared by the process according to the invention does not dissolve inthe substrate under conventional industrial operating conditions.

EXAMPLE 75

Cross-linked, crystalline isomerase, isomerase bound to DEAE celluloseand original free soluble isomerase were compared with each other bykeeping them under identical chemical and physical conditions. Theconditions were chosen so that each enzyme sample lost its activity to ameasurable extent in a reasonably short time, i.e., in 10-30 hours. A 5g portion of each enzyme preparation was mixed into 150 ml of 0.05Msodium phosphate buffer (pH 6.0), which further contained 1.5 mmol/literMgS04 and 2 mmol/liter NaHS0₃. The mixture was shaken for several hoursat 70° C. Samples were taken intermittently from the mixture, and theactivity of the remaining isomerase was determined from the samples. Thehalf-value time of each enzyme sample (i.e., the time period duringwhich the activity drops to one half of the original value) wasdetermined on the basis of the activity decrease. The results are shownin the following Table IX.

                  TABLE IX    ______________________________________    Enzyme sample        Half-time (h)    ______________________________________    Original soluble isomerase                         2.2    Isomerase bound to DEAE cellulose                         3.3    Cross-linked crystalline isomerase                         19.0    ______________________________________

It appears from the results that cross-linked crystals retain theirenzymatic activity substantially better than a free enzyme or an enzymeimmobilized in a known manner.

EXAMPLE 76

Cross-linked crystalline isomerase and isomerase bound to DEAE cellulosewere packed into a cylindrical column reactor similarly as described inExample 70. Glucose solution was pumped continuously through thecolumns. The glucose solution had the same composition as in Example 70except that the pH was adjusted to 6.0. The temperature of the columnswas 60° C. during the test. The activity of the enzyme contained in thecolumns was calculated on the basis of the flow rate and the fructosecontent of the solution which has flown through. The results are shownin the following Table X.

                  TABLE X    ______________________________________                         Activity half-    Reactor packing      time (h)    ______________________________________    Cross-linked isomerase crystal mass                         120    Isomerase bound to DEAE cellulose                         36    ______________________________________

It appears from the results that cross-linked crystals retain theirenzymatic activity excellently.

EXAMPLE 77

Samples of typical industrially used isomerases and the crosslinkedcrystals of this invention were packed into similar laboratory columns.Crosslinking of the crystals was performed as described in Example 51.The inside diameter of the cylindrical columns was 2.7 cm and height 50cm. The were water jacket thermostated to 60° C. The lower end of thevertical columns had a screen to keep the enzyme granules in the columnand to let the sugar solution flow through. Twenty grams (on a drysubstance basis) of each enzyme was poured into the individual columns.Glucose substrate having the composition as in Example 70 was pumpedwith an adjustable laboratory pump through the columns. Each column hadan individual pump. The temperature of the substrate was adjusted to 60°C. The product coming through the column was assayed for fructose andglucose content. It was observed that each different enzyme preparationproduced a different content of fructose when the substrate flow ratewas similar. The flow rate of each column was adjusted individually bytrial and error until the fructose content of each product was 45percent of total sugars (glucose in fructose). The following table liststhe enzymes and corresponding flow rates to produce 45 percent fructose.

                  TABLE XI    ______________________________________    Enzyme             Substrate Flow    (20 g dry substance)                       (milliters per hour)    ______________________________________    SPEZYME IGI, commercial                       179    product of Finnish Sugar Co.    SWEETZYME T, commercial                       197    product of NOVO Co. Denmark    TAKASWEET, commercial                       94    product of Miles-Kali Chemie    Crosslinked isomerase crystals                       1364    of diameter 100-200 micrometers    Crosslinked isomerase crystals                       1121    of diameter 500-600 micrometers    Crosslinked isomerase crystals                       1098    of diameter 900-1100 micrometers    ______________________________________

The flow rates of the commercial samples represent what is typicallyobserved in present industrial practice. The flow rates of the crystalcolumns were substantially higher. In industrial practice, the highactivity of crosslinked crystals will result in essentially small enzymecolumns, which will give savings in investment and processing costs.

I claim:
 1. A process for isomerizing a substrate selected from thegroup consisting of glucose, fructose, and xylose, the processcomprising contacting said substrate with a crystalline cross-linkedglucose isomerase wherein said crystalline cross-linked glucoseisomerase is produced by the steps of a) adding to a crystalline glucoseisomerase suspension a compound containing at least one amino group,said compound selected from the group consisting of an ammonium salt,lysine, and tryptophan, said compound being added at a concentration ofbetween 1% and about 15% and at a pH of about 7.0 to about 8.4; b)adding glutaraldehyde to initiate a cross-linking reaction; and c)continuing the cross-linking reaction until cross-linking occurs.